A&A 381, L29-L32 (2002)
DOI: 10.1051/0004-6361:20011604
A. Omar1 -
K. R. Anantharamaiah1,
-
M. Rupen2 -
J. Rigby3
1 - Raman Research Institute, C.V. Raman Avenue, Bangalore, 560 080, India
2 -
National Radio Astronomy Observatory, Socorro, NM, USA
3 -
Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
Received 3 September 2001 / Accepted 12 November 2001
Abstract
VLA observations of OH absorption towards the elliptical galaxy NGC 1052
are presented. Both OH lines, at 1665 and 1667 MHz, were detected in
absorption towards the center of NGC 1052. The hyperfine ratio of the two OH
lines (1667/
1665) is
as compared to 1.8
expected for the excitation under LTE conditions for an optically thin
cloud. The column density of OH is estimated to be
cm-2 assuming
K. The
centers of both the OH lines are redshifted from the systemic velocity of
the galaxy by
173 kms-1. The velocity of OH line coincides with
the velocity corresponding to the strongest HI absorption. We suggest that
OH absorption is arising from a molecular cloud falling towards the nucleus.
The OH line, though narrower, is found to be within the much broader and
smoother H2O megamaser emission. The possible link between OH/HI and
H2O emission is discussed.
Key words: galaxies: active - galaxies: individual: NGC 1052 - galaxies: ISM - radio lines: galaxies
NGC 1052, a moderately luminous (
)
elliptical
galaxy of type E4, is a member of a small group in the Cetus-I cloud. There
are several estimates of the velocity for this system in the literature,
which differ from each other by a few tens of kms-1. We adopt
kms-1, estimated from the optical emission lines
(de Vaucouleurs 1991), which implies that NGC1052 is at a distance of 21 Mpc
(assuming H0=70 kms-1Mpc-1 and q0=0). It is classified as
a LINER (Fosbury et al. 1978; Ho et al. 1997) and is known for its several water megamasers
(Braatz et al. 1996; Claussen et al. 1998). HI absorptions, redshifted from the systemic
velocity, were detected at 1486, 1523 and 1646 kms-1 against the
nuclear continuum source (van Gorkom et al. 1986). NGC 1052 was reported to have CO emission as well as absorption by Wang et al. (1992), but later observations by
Wiklind et al. (1995) failed to confirm those detections. More recently, Knapp & Rupen (1996)
have reported a possible CO absorption from NGC 1052 near 1622 kms-1.
Since the reported CO detections are quite noisy, it remains uncertain
whether NGC 1052 has a molecular component associated with the HI (21 cm)
absorption.
Here we report the first detection of 1665 and 1667 MHz OHabsorption in NGC1052. The next section describes the observational details and results. Subsequent sections compare these results with observations at optical, X-ray, and other wave bands, and discuss some of the implications.
Parameter | Value |
---|---|
Date of observation | 1998 Sep. 03 |
RA, Dec (J2000.0) | 02 41 04.79, -08 15 20.75 |
Observing duration (hrs) | 5 |
Range of baselines (km) | 0.1-11 (B config) |
Observing frequencies (MHz)(IF1,IF2) | 1656.50, 1658.30 |
Bandwidth per IF (MHz) | 1.562 |
Number of spectral channels | 64 |
Polarizations | RCP & LCP |
Synthesised beam (Natural Weight) |
![]() ![]() |
Velocity resolution | 4.4 kms-1 |
Frequency resolution (kHz/channel) | 24.4 |
Amplitude calibrator | 0137+331 (3C 48) |
Phase calibrator | 0240-231 |
Bandpass calibrator | 0319+415 (3C 84) |
rms noise per channel (mJy beam-1) | 0.7 |
The core/jet morphology in the continuum image of NGC1052 is in accordance
with the previous observations by Jones et al. (1984). The peak continuum
flux density of the core is 1.14 Jy. The total flux density including
contributions from the two radio lobes is
1.23 Jy. The continuum
image (Fig. 1) shows that the radio axis is at a position
angle (E to N) of 103
.
The two radio lobes are asymmetrically located
about the radio nucleus, being 14
to the east and 8
to the
west. The continuum nucleus and the line absorption are unresolved with the
synthesised beam (
,
PA
). Both 1665 and
1667 MHz lines are detected at a redshifted velocity of
173 kms-1 with respect to the systemic velocity of the galaxy. The column
density of OH can be estimated from
The AIPS gaussian fitting routine "SLFIT'' was used to derive the line
parameters. The peak optical depth of the 1667 MHz line is
and that of the 1665 MHz line is
.
The FWHM of 1667 and 1665 MHz lines are
and
kms-1 respectively. Given the
uncertainity in the overall shape of the 1665 MHz line due to low optical
depth, profiles of the 1665 and 1667 MHz lines can be considered similar.
The ratio of the integrated optical depth is
which is marginally
higher than that expected (viz. 1-1.8) for excitation in thermal
equillibrium. The mean value of 1667 to 1665 MHz line ratio is about 1.6 for
galactic diffuse clouds (Dickey et al. 1981).
![]() |
Figure 1:
The radio continuum image of NGC1052 drawn as contours with levels
of 1.8 mJy beam
![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 2: A plot of the optical depth of 1667 and 1665 MHz absorption lines towards the nucleus of NGC1052. The spectrum has been Hanning smoothed offline using a window of 3 adjacent channels. The figure displays the entire velocity coverage by VLA observations. The region marked by dashed lines in 1665 MHz spectrum was affected by interference. The velocity range over which HI absorption and H2O masers are observed are indicated in the top and bottom frames respectively. The systemic velocity is indicated on top left corner of the upper frame. |
Open with DEXTER |
The gas is expected to be much hotter in the vicinity of an AGN due
to enhanced Ly
pumping which in turn will increase the
to a
few thousand kelvin. Assuming,
K, the predicted total
N(HI) will be
cm-2 including all three HI
components. For the detected OH component, taking the relative abundance
ratio of OH/H
(Guèlin 1985; Liszt & Lucas 1999), the implied column
density of H2 is
) cm-2. The
implied CO column density is about
cm-2, which is
about 10 times higher than predicted from CO observations. In comparison,
X-ray observations indicate a hydrogen column density greater than
cm-2 (Weaver et al. 1999), which is significantly higher
than the total hydrogen column estimated via radio observations (HI & OH).
This excess column density inferred from X-ray data has been seen in many
active galaxies, and, was explained due to excess absorption by a
combination of dust and partially ionized gas (Gallimore et al. 1999). It should be
noted here that since HI and OH absorptions are spatially unresolved, the
estimated values of OH and HI column densities are only a lower limit.
Also, X-ray absorption is arising towards the nucleus which is free-free
absorbed at wavelengths corresponding to the HI and OH absorptions
(Kameno et al. 2001), therefore, radio observations are sampling off nuclear gas
which may be of different composition than the gas probed via X-ray
observations.
It is very surprising that the OH absorption, though narrower than the water
maser emission, is coincident with the velocity centroid of the 22 GHz
H2O masers. NGC1052 is the only known elliptical galaxy having H2O megamaser emission. The megamasers and their link with AGNs are generally
understood in terms of obscuring torus models. The link is thought to be a
consequence of irradiation of the inner face of the torus by hard X-rays
from the nuclear continuum source, which enhances the water abundance within
a molecular layer at a temperature of 400-1000 K (Neufeld et al. 1994). H2O megamasers of NGC1052 are unusual in showing a relatively smooth profile
which moves in velocity over time by about 70 kms-1 on a time scale of
a year (Braatz et al. 1996). Water masers in NGC1052 are distributed along the
jet rather than perpendicular to it (Claussen et al. 1998) unlike in NGC 4258 in
which water masers are originating in a torus (see Miyoshi et al. 1995). Claussen
et al. (1998) suggested that these masers are excited by shocks in to
circumnuclear molecular cloud, or alternatively, amplifying radio continuum
emission of the jet by foreground molecular clouds. It should be noted that
the shocks can also enhance the abundance of OH by dissociation of H2O
before the gas is cooled down below 50 K (Wardle 1999), however, the
observed column density of OH is one order of magnitude less than that
predicted. A drift in the velocity of maser feature was considered as a
consequence of the moving jet which will illuminate different parts of the
foreground H2O masing cloud. Efficient maser emission will take place at
total column density ()
below the quenching density which is estimated
as
1025-1027 cm-2 for NGC1052 (see Weaver et al. 1999). This
upper limit on column density is well above than that predicted from our
observations. However, it is not clear how HI/OH are quite stable over a
long period of time while H2O emission changes substantially over a short
time scale. Further simultaneous observations of HI, OH and H2O masers
are required to make a connection between molecular gas traced by OH
absorption and H2O masing gas.
These VLA observations have resulted in the first detection of OH absorption
in an elliptical galaxy. Both, 1665 and 1667 MHz OH absorption, were
detected from the elliptical galaxy NGC1052. The linewidths of both the OH lines are significantly large as compared to that expected for a cloud in
thermal conditions at few tens of K. The gas is predicted to be close to the
nucleus. A remarkable coincidence of velocity is found with the strongest
and redshifted HI absorption and H2O emission, however link to the
megamaser emission is still not understood. Based on the abundance ratio of
OH/H2 as
,
it is predicted that the column density of
molecular gas in NGC1052 is comparable to HI. Higher angular and spectral
resolution observations would be usefull for detail kinematics of the OH
absorption while simultaneous observations of H2O and HI/OH observations
would be neccessary to understand the link between masing gas and molecular
gas traced by OH absorption.
Acknowledgements
The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.